United Nations education envoy Gordon Brown appealed on Tuesday for $500 million to allow half of the two million Syrian children who are refugees in Lebanon, Turkey and Jordan to go to school and offer their families an alternative to fleeing to Europe.

The former British prime minister said the aim was then to get all two million refugee children in school in 2017.

Brown said that while some 400,000 Syrian children have made their way to Europe and other countries to escape Syria's five-year civil war, there are still some two million refugee children in Lebanon, Turkey and Jordan.

"Unless we take action to deliver educational opportunity then we have thousands of children on the streets, they are vulnerable to child trafficking, they are vulnerable, obviously, to radical extremism," Brown told reporters.

He said they were also at risk of child labor and rates of child marriage were also increasing.

Brown said that using a "double-shift" system, some 207,000 Syrian children were now being taught in schools in Lebanon in the afternoon and evenings after Lebanese children had completed their classes earlier in the day.

"This is a ray of hope for children, the majority of whom have not had the chance of education since they left Syria. It gives families a choice now where they thought previously they had no alternative but to leave for Europe," Brown said.

The aim was to have some 400,000 Syrian children in Lebanon in school this year, along with 400,000 Syrian children in Turkey in school - up from 200,000 - and 200,000 Syrian children in Jordan - up from 130,000 - he said.

"Over these next few weeks the pressure is on to raise an additional $500 million to ensure that a million children who are refugee children are in school," Brown said, adding that in 2017 "we want to be able to say that every single refugee child is being offered, boy and girl, the chance of education."